1. Field of the Invention
The present invention relates to a data transmission system utilizing a power line. More specifically, the present invention relates to an improvement in a data transmission system utilizing a power line, wherein a transmitter and a receiver are coupled to a power line, control data is transmitted from the transmitter and is received by the receiver, whereby a load provided in the receiver is controlled and data representing a control state of the load is then transmitted from the receiver to the transmitter.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a data transmission system utilizing a power line which constitutes the background of the present invention. FIG. 2 is a graph showing a level of a transmission data in the FIG. 1 data transmission system.
Referring to FIGS. 1 and 2, a conventional data transmission system utilizing a power line will be described. As shown in FIG. 1, a transmitter 1 and a receiver 2 are coupled to a power line 3. The transmitter 1 transmits control data including a plurality of bits in the form of high frequency signal of say 100 kHz superim posed on an alternating current of a commercial power supply. The receiver 2 receives the control data, thereby to control an apparatus being controlled such as a relay and transmits in a return manner return data representing a control state of the apparatus being controlled to the transmitter 1. Such data transmission system utilizing a power line is more fully disclosed in the co-pending U.S. patent application, Ser. No. 200,079, now U.S. Pat. No. 4,377,804, entitled "Data Transmission System Utilizing Power Line", filed Oct. 24, 1980 by Yoshiharu Suzuki and assigned to the same assignee as the present invention. Therefore, the above described application is incorporated herein by reference thereto.
In the case of such data transmission system, the power line 3 has an inherent resistance and also has a load 4 of a capacitive nature coupled. Therefore, the level of the control data transmitted from the transmitter 1 is decreased as a result of the influence by such resistance and the capacitive load 4. More specifically, if and when the distance between the point A where the transmitter 1 is installed and the point B where the receiver 2 is installed is large, the level of the data transmitted from the transmitter 1 becomes smaller than a level receivable by a receiver 2 before the data reaches the receiver 2. Therefore, it could happen that an apparatus being controlled coupled to the receiver 2 cannot be controlled, in the case where the distance between the transmitter 1 and the receiver 2 is large. Similarly, the data representing a control state of the apparatus being controlled cannot be returned from the receiver 2 to the transmitter 1.
In order to eliminate such problems, it is necessary to take any of the approaches in which the output level of the data transmitted from the transmitter 1 is increased, the reception sensitivity of the receiver 2 is increased and an attenuation amount of the level of the data in transmission over a power line is decreased. However, an increase of the output level of the transmitter 1 threatens to cause an electric wave interference upon other machines inasmuch as the carrier wave of the control data is as relatively high as 100 kHz. On the other hand, an increase of the reception sensitivity of the receiver 2 could cause the receiver 2 to receive even a noise other than the data, with the resultant fear of malfunction by the receiver 2. Thus, in order to transmit the control data in a proper level in an increased distance between the transmitter 1 and the receiver 2, it is considered most appropriate to decrease an attenuation amount of the level in transmission over the power line 3. However, a decrease of an inherent resistance of a power line or a capacitive load 4 entails another difficult problem that a particularly designed power line need to be utilized.